Conventionally, a water pump used in a water-cooled engine includes an impeller provided in one end of a rotary shaft supported to a body through a bearing to rotate the impeller for circulating cooling water through the engine. For instance, a water pump disclosed in Patent Document 1 includes a space defined between a mechanical seal and a bearing, in which a few waterdrops entering the space through the mechanical seal are discharged to a reservoir provided in a lower portion of the body through a water vent to allow cooling water to flow out from a drain for establishing communication between the reservoir and the air.
Further, a water pump disclosed in Patent Document 2 includes a groove provided in one side of a reservoir for guiding cooling water, a plug provided in the other side of the reservoir to close the one side of the reservoir, and a drain provided above the plug for establishing communication between the reservoir and the air.